1. Field of the Invention
This invention relates to a method of manufacturing ball grid arrays and the ball grid array.
2. Brief Description of the Prior Art
One form of packaging for semiconductor devices is known as a ball grid array (BGA) package. Such semiconductor packages involve the use of small eutectic solder balls (generally about 0.012 inch in diameter and generally a tin/lead alloy) deposited on a surface of the package which are coupled to circuitry within the package and present a solderable array surface to a circuit board. The solder balls are each attached or placed on the package surface by soldering the balls to a layer of gold generally having a thickness of from about 20 to about 40 microinches. The gold layer rests over a layer of nickel having a thickness generally of about 200 microinches which rests over a copper pad having a thickness of about 0.2 millimeters. The copper pad is on a surface of a dielectric substrate with the combination of the copper, nickel and gold referred to as a plated solder ball attach pad or site. The substrate on which the pad rests is generally a ceramic, laminate or polyimide tape or similar substrate which is the support for the ball grid array. In most cases, the opposite surface of the substrate has metal pads for either flip chip die mounting or wire bonding. These metal pads are connected to the solder ball pads by vias extending through the substrate or the like. In some cases, the solder ball attach pads and the wire bond pads are on the same side of the substrate.
The solder balls must be soldered essentially two times. The first soldering operation takes place during the attachment of the solder balls to the package (i.e., to the solder ball attach pads on the substrate) during package assembly and the second soldering operation involves reflow of the solder balls to the circuit board during device mounting.
A recurrent problem in ball grid array devices is that of adhesion of the solder balls to the package substrate. It is postulated that this problem may, in fact, be caused by gold embrittlement of the solder ball in a narrow region within the solder ball. This problem may occur either by having too much gold on the substrate or by having insufficient heat, thereby not allowing the gold to diffuse fully into the solder ball. Either condition can lead to a brittle region which is caused by having a high concentration of gold in the solder within the ball which can result in cracking and separation of the solder ball from the substrate. This separation can lead to a variety of problems such as, for example, open or faulty circuit connections.